In general, a cellular radiotelephone includes a transceiver for transmitting and receiving radio communications to and from a radio base station, a controller for controlling the transmission and reception of the radio communications, and a user interface. More particularly, the user interface can include a keypad for accepting data input from a user and a visual display (such as a liquid crystal display) for providing information to the user. Furthermore, many cellular radiotelephones are battery operated allowing mobility during use.
In addition, backlighting can be used to illuminate the user interface. For example, one or more light emitting diodes (LEDs) can be used to provide backlighting to the user interface. In particular, keypad backlighting has been implemented using arrays of yellow-green (570 nm) light emitting diodes (LEDs). An array including a plurality of pairs of light emitting diodes (LEDs) 33 has been used wherein each of the LEDs in a pair are connected in series and wherein each of the series connected pairs of light emitting diodes are connected in parallel as shown in FIG. 1. The six parallel light emitting diode circuits are switched ON or OFF through the common NPN transistor 21.
The current through the collector of the common NPN transistor 21 is controlled using the voltage reference made up of the resistors 23 and 25, and the diode 27. A resistor 29 is also provided between the emitter of the transistor 21 and ground. Furthermore, a resistor 31 is connected in series with each of the pairs of series connected LEDs 33. As will be understood by one having skill in the art, the voltage at the base of the transistor 21 can be determined using the formula: EQU V.sub.BASE =V.sub.BE +V.sub.R
where V.sub.BASE is the voltage at the base of transistor 21, V.sub.BE is the voltage between the base and the emitter of transistor 21, and V.sub.R is the voltage across the resistor 29. Increasing the collector current will thus increase V.sub.R thereby reducing V.sub.BE and limiting the collector current. The transistor 21 thus acts as a simple current source and operates in the linear forward active region of the transistor.
The collector current may be affected by a number of variables including the output impedance of the BACKLIGHT source signal; the process variations and temperature dependence of the forward voltage of diode 27; the process variations and temperature dependence of V.sub.BE ; and the temperature coefficients of and tolerances of resistors 23 and 25. Given these uncontrolled process and environmental variables, the collector current through transistor 21 may be unreliable without allowing for relatively wide tolerances.
When using a 5 cell rechargeable battery to operate the cellular radiotelephone, sufficiently wide tolerances may be available. In general, a 5 cell rechargeable battery has a typical operating voltage of 5.0V to 7.0V. This operating range may provide ample voltage over the life of the battery to overcome the forward voltage (V.sub.F) Of the two series LEDs 33, the collector-emitter voltage of the NPN transistor 21, and the voltage across the degeneration resistor 29. Assuming that the saturation current (V.sub.SAT) through transistor 21 is 200 mV, and ignoring the effects of the degeneration resistor 29, the minimum battery voltage required to guarantee backlighting can be calculated as follows: EQU V.sub.RD +2(V.sub.FD)+V.sub.CE =0.0V+2(2.2V)+2.2V+0.2V=4.6V.
where V.sub.RD is the voltage across the diode resistor 31, V.sub.FD is the forward voltage across one of the LEDs 33, and V.sub.CE is the collector to emitter voltage of the transistor 21.
The forward voltage V.sub.FD of a light emitting diode (LED) 33 is dependent on the conduction current through the LED, the ambient temperature, and the process variations from diode to diode. Accordingly, the LED forward voltage is typically less than the 2.2V listed in the manufacturer data sheets. FIG. 2 is a graph illustrating data collected in the laboratory using the backlighting circuit of FIG. 1 implemented with six pairs of series connected LEDs with the LED pairs being connected in parallel wherein each of the LEDs is a yellow-green 570 nm LED. The data used to generate this graph is provided below in Table 2.
TABLE 2 V I V I V I V I V I 80 dgs 60 dgrs 25 dgrs 0 dgrs (-)30 dgrs 10 3.548 10.357 3.604 10.091 3.729 10.308 3.819 10.42 3.94 10.15 20 3.653 20.594 3.708 20.291 3.821 20.32 3.905 20.312 4.03 20.55 30 3.727 30.522 3.782 30.555 3.886 30.05 3.968 30.408 4.09 30.301 40 3.788 40.079 3.84 40.13 3.94 40.413 4.018 39.801 4.15 40.599 50 3.846 50.413 3.894 50.012 4 50.815 4.07 50.808 4.2 50.275 60 3.899 60.461 3.95 60.645 4.04 60.44 4.11 60.399 4.25 60.563 70 3.95 70.314 3.992 70.141 4.08 70.033 4.15 70.105 4.3 70.559 80 3.99 80.111 4.04 80.213 4.13 80.06 4.19 80.601 4.34 80.665
The voltage difference between the circuit input V.sub.SWDC and the voltage at the collector of the transistor 21 was measured for various collector currents and temperatures for applications designed for a diode conduction current in the range of 8 mA to 12 mA (48 mA to 72 mA total) for a typical radiotelephone operating according to the DAMPS standard using a 5 cell rechargeable battery. As shown, a minimum voltage of 4.3 volts may be required to maintain forward conduction at cold temperatures in the range of -300.degree. C. These curves also indicate that the compliance limits of the circuit may be exceeded as the voltage drops below 4.3V thereby reducing the current through the LEDs. In this condition, the user may notice keypad backlight dimming or "brownout".
The backlighting circuit of FIG. 1 may provide acceptable performance for a radiotelephone powered by a 5 cell rechargeable battery as discussed above. This backlighting circuit, however, may not provide acceptable performance when used in a radiotelephone powered by a 4 cell NiCD/NiMH rechargeable battery which may provide a normal operating voltage in the range of 4.0V to 5.7V with an "end-of-life" voltage set at 4.2V. A typical discharge curve for a 4 cell battery is illustrated in FIG. 3. As shown, the end-of-life voltage is set at 4.2V.
Assuming that the saturation voltage of transistor 21 is 200 mV and assuming that there is a 4.3V drop across the LED array, a minimum of 4.5V is required to guarantee consistent backlighting operation. The LEDs would thus provide relatively consistent lighting at the upper end of the battery voltage range, but the LEDs could be expected to fade or turn off as the battery voltage drops below 4.5V. Furthermore, LED fading could be expected to occur at higher battery voltages in low temperature conditions and/or with LEDs having less than the average forward voltage as a result of standard process variations.
Raising the "end-of-life" voltage setting can reduce the occurrence of backlight brownout. For example, the nominal "end-of-life" voltage can be set to 4.6V to provide consistent backlighting operation. As shown in FIG. 3, however, this approach could reduce the useful operating time for the battery by as much as 25%.
Alternately, the LED array can be arranged with all of the LEDs in parallel thereby reducing the voltage drop across the LED array. This arrangement, however, may double the current consumed by the backlighting circuit and double the heat generated thereby. The power consumed by the backlighting circuit is thus undesirably increased. Accordingly, there continues to exist a need in the art for improved backlighting circuits.